As processor operating power supply levels are lowered to reduce power consumption of the processor, performance of input-output (I/O) transmitters of the processor is adversely affected. The term “performance” herein refers to signal integrity attributes of a signal such as output signal swing of the signal driven by a transmitter, slew rate of the signal driven by a transmitter, eye width and height of the output signal swing, etc.
For example, for advanced CMOS process technologies of 22 nm or 14 nm, the operating voltage of the processor is scaled down below 0.9V. At such low power supply levels, the I/O transmitters generally cannot meet their specifications of performance parameters discussed above. One possible means for meeting the performance parameters is to provide dedicated higher power supplies to the I/O transmitters than the power supply provided to the rest of the core processor. However, such solutions are expensive and require dedicated power supplies and voltage regulators.
I/O transmitter (Tx) drivers are generally of two categories—current mode Tx driver and voltage mode Tx drivers. Current mode Tx driver, such as a current mode logic (CML), suffers from headroom degradation at lower power supply levels. The term “headroom” herein refers to the amount of power supply level needed to keep the current source of the current mode Tx to remain in the transistor saturation region. As power supply level reduces, the current source of the current mode Tx suffers from reduced capability to remain in saturation mode. Lower power supply levels for current mode Tx driver also cause degraded output impedance of the current source of the current mode Tx driver. While the output impedance may be improved by increasing the size (W/L) of the current source of the current mode Tx driver, a larger current source results in higher parasitic capacitances and thus reduced output voltage swing.
The term “voltage swing” herein refers to the peak-to-peak voltage difference of the signal driven by a driver of a Tx measured from the logical low level to the logical high level of the output signal.
FIG. 1 is a voltage mode I/O Tx driver 100 that exhibits low power supply rejection ratio (PSSR) with limited output signal swing. The driver 100 consists of a pull-up device Mp1, a pull-down device Mn1, and an output impedance R. The output signal swing at the output node Vo is directly proportional to the supply voltage. As power supply voltage level is reduced, the output signal swing reduces. A reduced output signal swing results in narrower eye width and height and degradation of other performance parameters.